Near-circular arcs generate empty paths when rounding is used

[jasondavies]

I encountered this edge case when using d3-shape:

d3.arc().innerRadius(55).outerRadius(75).startAngle(0).endAngle(6.283179980513967)()
"M0,-75A75,75,0,1,1,0,-75L0,-55A55,55,0,1,0,0,-55Z"

Now, d3.arc uses digits(3) by default, which means it's using d3-path with rounding.

Since there's no ambiguity when using d3-path's arc for a complete or near-circle (unlike arcTo which is undefined for coincident start/end points), and complete circular arcs are supported, I think this is a bug in d3-path, rather than d3-shape.

I traced it back to the rounding used in d3-path: nothing special is done to check for coincident points after rounding when drawing arcs.

[jasondavies]

Arguably it's simple enough to work around by disabling rounding for d3.arc, in which case it's probably fine for d3-path to rely on the 2π angle check and generate two arc commands as it does currently. However, rounding is on by default for d3.arc.